The present invention relates to a developing device provided with a developing roller that bears developer including magnetic carrier and toner on its circumferential surface, so as to develop a latent image formed on an image bearing member, an image forming apparatus that is provided with the developing device, and the developing roller that bears the developer including magnetic carrier and toner on its circumferential surface, so as to develop the latent image formed on the image bearing member.
In an image forming process of the electro-photographic method, which employs two component developing agent (hereinafter, also referred to as developer) including magnetic carrier and toner, a latent image formed on a photoreceptor drum (image bearing member) is developed by applying toner included in the developer bone by the developing roller, so as to form a visible toner image on the photoreceptor drum concerned.
Generally speaking, the developing roller is constituted by a developing sleeve made of a non-magnetic material and a magnetic member provided with a plurality of fixed magnetic poles, to convey the developer to a developing magnetic pole disposed opposite to the photoreceptor drum so as to make the toner adhere onto the photoreceptor drum from the developing magnetic pole serving as one of the plurality of fixed magnetic poles. Further, the developer passed through the developing magnetic pole are peeled off the developing sleeve by the peeling action of a peeling magnetic pole, serving as another one of the plurality of fixed magnetic poles, and, after being sufficiently agitated, the developer are made to adhere onto the circumferential surface of the developing sleeve by an adhering action of a developer-layer forming magnetic pole, serving as still another one of the plurality of fixed magnetic poles.
Incidentally, the size minimization trend in the field of image forming apparatus that employs the electro-photographic process, including a copier, a printer, etc., has been considerably progressed in the recent years. Due to an influence of such the size minimization trend, the developing device that includes the developing roller has been minimized as smaller as possible. The problem to be arisen associating with the size minimization of the developing roller is a releasability of developer. The smaller the outer diameter of the developing roller is, the shorter the distance between the peeling magnetic pole and the developer-layer forming magnetic pole becomes. As a result, it becomes impossible to peel a sufficient amount of developer off the peeling magnetic pole, and accordingly, a residual amount of developer is attracted to the developing roller as it is. Therefore, it is impossible to supply the developer including a sufficient amount of toner to the developing magnetic pole, resulting in the density deterioration of the toner image to be formed on the photoreceptor drum.
To cope with the abovementioned problems, there has been set forth such a technology that adjusts the density of magnetic flux generated by the plurality of fixed magnetic poles provided in the magnetic member, so as to prevent the toner image from generating density deterioration.
For instance, according to the technology set forth in Tokkaihei 11-161029 (Japanese Non-Examined Patent Publication), the magnetic poles arranged on the circumferential surface of the developing roller from the conveyance magnetic pole (equivalent to the peeling magnetic pole) to the adhering magnetic pole (equivalent to the developer-layer forming magnetic pole) are set at the same polarity. Further, the magnetic flux density A at such a position that is rotated at 10° from the position of the adhering magnetic pole side 100G of the conveyance magnetic pole, provided on the circumferential surface of the developing roller, in the reverse direction of the developing roller rotational direction is made to fulfill the Equation of (A−100)/10≦16, and the magnetic flux density B at such a position that is rotated at 10° from the position of the conveyance magnetic pole side 100G of the adhering magnetic pole, provided on the circumferential surface of the developing roller, in the developing roller rotational direction is made to fulfill the Equation of (B−100)/10≦15. In other words, Tokkaihei 11-161029 sets forth such the technology that, considering the releasability of developer, the magnetic flux density around the peripheral space of the specific fixed magnetic pole is adjusted to a predetermined value.
According to the technology set forth in the Tokkaihei 11-161029, however, merely the magnetic flux density around the peripheral space of the conveyance magnetic pole (equivalent to the peeling magnetic pole) and/or around the peripheral space of the adhering magnetic pole (equivalent to the developer-layer forming magnetic pole) is adjusted, and the magnetic flux density at the conveyance magnetic pole becomes high. As a result, since the attractive force toward the center of the developing roller at the conveyance magnetic pole is getting large, the developer residing at that portion are attracted toward the developing roller, resulting in insufficient releasability of the developer.